Circuit pattern formation using metal nanoparticles having a size of nanometer order is known, and a number of techniques have recently been proposed in which a circuit is drawn or printed directly on a variety of substrates using a conductive metal ink (a metal ink containing metal nanoparticles) by means of an inkjet printer or a dispenser, and the applied ink is fired to form conductor wiring or electrodes.
Circuit pattern formation on various substrates by firing metal nanoparticles at low temperature is exemplified by the proposal of JP 2002-334618A. Circuit formation by inkjet printing a conductive metal ink is disclosed in JP 2002-324966.
The technique in which a circuit pattern is directly printed on a substrate by inkjet printing or the like is attracting attention as a promising process allowing for great reduction of production cost because of a fewer number of steps involved and less waste from the steps than the generally spread, conventional circuit pattern forming technique utilizing photolithography. The conventional technique relying on photolithography is exemplified by the one disclosed in JP 9-246688A.
We have thus seen the technological changes from photolithographic techniques into inkjet printing or dispenser techniques, which have enabled more convenient and less expensive circuit formation on a substrate.
Nevertheless, the circuit formation technique using a conductive ink by inkjet printing or dispensing using a dispenser (hereinafter referred to as a dispenser method) has not been widespread primarily for the following reasons.    (i) The resulting conductor film lacks adhesion to various substrates, failing to fulfill fundamental characteristics required of a circuit board.    (ii) The resulting conductor film lacks surface smoothness. A circuit usually has a laminate structure including a base layer, so that it would have limited applicability in various respects unless the conductor film has sufficient surface smoothness. For is example, a different component layer provided on a rough surface of a conductor film would be influenced by the underlying surface roughness and fail to maintain good thickness uniformity.
The above-mentioned problem (i) is largely caused by the characteristics of a dispersion medium used in a conductive ink. It is considered that the adhesion of a conductor film formed by sintering and solidification to a substrate is governed by the chemical reaction between a binder component present in the dispersion medium and the substrate. The problem (ii) is believed to be caused by the characteristics of both the metal powder (metal particles) and the dispersion medium constituting a conductive ink. It is obvious that coarse metal particles make it impossible to form a conductor film with smooth surface. In addition to this, if the dispersion medium vigorously vaporizes and escapes from the inside of the conductor film during firing, it easily results in a failure to form a smooth film surface.
Thus, a conductor film formed by use of a conductive ink has been required to have good adhesion to various substrates and as smooth a surface as possible. A conductor film formed by using a nickel ink, in particular, has been required to have an average surface roughness (Ra) of 10 nm or smaller and a maximum surface roughness (Rmax) of 200 nm or smaller in view of its application fields.